Polymers, especially thermoplastic polymers, are usually manufactured in the form of pellets or powders. In order to manufacture articles from such polymers the pellets or powders are usually heated so as to convert the polymers to a viscous, especially molten, state and the resultant polymer is then fabricated into articles of the desired shape. Such fabrication of articles from thermoplastic polymers may be accomplished using, for example, extrusion, injection moulding or rotational moulding techniques.
Polymers are susceptible to oxidation. The oxidation of polymers is temperature-dependent, occurring more rapidly at elevated temperatures. Oxidation of a polymer, which may occur by a number of mechanisms depending on the particular polymer and the conditions to which the polymer is subjected, affects the properties of articles fabricated from the polymer. The amount of oxidation of a polymer during the manufacture of articles from the polymer is frequently important in determining the subsequent useful life of the article in a particular end use.
Moreover, the effects of oxidation of polymers, especially the effects resulting from oxidation during the manufacture of articles from the polymer, may not become apparent in the properties of the article for months or even years after the manufacture of the article.
In view of the importance of minimizing the amount of oxidation occurring at elevated temperatures during the manufacture of articles from a polymer, stabilizers especially those stabilizers known as antioxidants are incorporated into the polymer. The type and amount of stabilizer depends in particular on the type of polymer, the processing conditions used in the manufacture of articles and the end use for the articles. Mixtures of stabilizers are sometimes used especially if the mixture has synergistic antioxidant effects. In order, for example, to evaluate the effectiveness of a particular antioxidant in a polymer there is a need for apparatus in which the oxidation of polymers may be evaluated in a relatively short period of time.
A large number of techniques have been used in attempts to evaluate the oxidative stability of polymers and the effectiveness of antioxidants in polymers in relatively short periods of time. For example, the oxidation of polymers may be studied using oven-aging techniques in which polymer compositions, usually in the form of test samples, are maintained at elevated temperatures especially temperatures well below the melting point of the polymer. Periodic determinations of the physical and/or chemical properties of the polymer are made. Alternatively differential thermal analysis techniques may be used to study the degradation of polymers, as is described, for example, in U.S. Pat. No. 3,868,221 of J. B. Howard and R. F. Westover, which issued Feb. 15, 1975. The rate of oxygen up-take of molten polymers, especially under elevated pressures of oxygen, may be determined using manometric or volumetric techniques. The above techniques are generally characterized by the use of solid polymers or of molten polymers that are not subjected to shear conditions and by the use of samples of relatively small size. Correlations of the results obtained using such techniques and the oxidation actually occurring in the manufacture of articles using, for example, extrusion techniques may be somewhat tenuous.
The degradation of some polymers under so-called "milling" conditions may be determined according to the procedure of ASTM D-1243 or by monitoring the torque on shearing means while subjecting polymers to heat and shear.
Techniques for determining the oxidative stability of polymers and/or the effectiveness of antioxidants are discussed by W. L. Hawkins in "Polymer Stabilization" Wiley-Interscience 1972 especially pages 422-436 thereof.
In spite of the variety of techniques for monitoring the oxidation of polymers there is a need for apparatus capable of being used for determining such oxidation with greater precision and under conditions more closely resembling the condition under which polymers are fabricated into articles.